manage focus on discussion reply attach button
manage focus on discussion reply attach button

fixes CNVS-18667

test plan:
- on a discussion show page, navigate with the keyboard to reply, and
  then to the attach button
- hit enter, focus should land on the "choose file" button, not be reset
  to the top of the page
- tab to the 'remove attachment' red x icon, and hit enter
- focus should return to the attach link

Change-Id: Ie3bdb27ac1f1e8ce8659a05505b42c8bf3cf51a5
Reviewed-on: https://gerrit.instructure.com/49530
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>
QA-Review: Adam Stone <a0b2b61e2edcf99aa16080c2254a6ae6923d1fac@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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